1. Field
The disclosed concept pertains generally to systems for track circuits and, more particularly, to interlocking systems. The disclosed concept further pertains to overlay systems for such interlocking systems. The disclosed concept also pertains to methods for monitoring track circuits.
2. Background Information
In the art of railway signaling, traffic flow through signaled territory is typically directed by various signal aspects appearing on wayside indicators or cab signal units located on board railway vehicles. The vehicle operators recognize each such aspect as indicating a particular operating condition allowed at that time. Typical practice is for the aspects to indicate prevailing speed conditions.
For operation of this signaling scheme, the track is typically divided into cascaded sections known as “blocks.” These blocks can be electrically isolated from adjacent blocks by typically utilizing interposing insulated joints. In many audio frequency (AF) track circuits, continuous welded rail is used, and the blocks are delimited by tuned bonds. When a block is unoccupied, track circuit apparatus connected at each end are able to transmit signals through the rails within the block. Such signals may be coded to contain control data enhancing the signaling operation. Track circuits operating in this manner are referred to as “coded track circuits.” One such coded track circuit is illustrated in U.S. Pat. No. 4,619,425. When a block is occupied by a railway vehicle, shunt paths are created across the rails by the vehicle wheel and axle sets. While this interrupts the flow of information between respective ends of the block, the presence of the vehicle can be positively detected. Similarly, non-coded track circuits detect the presence of a railway vehicle via shunt paths.
In a track layout having a number of switch turnouts and rail crossings, it is necessary to assure a clear route, unobstructed by any other railway vehicles, for an entering train in order to fully exploit the train's speed capabilities. The concept of railroad interlocking, developed as early as 1857, provides this clear route assurance by preventing other vehicles from taking routes conflicting with that of the entering train.
Common interlocking systems make use of products and systems to detect occupancy (by railway vehicle(s)) of sections of track known as track circuits and employ relay or microprocessor-based logic to select the maximum speed that a railway vehicle can safely travel in a given area. With these systems, occupancy of track circuits plays an important role in the logical selection of the appropriate wayside signal aspect, wayside train-stopping device position, or onboard cab-signal aspect.
While track circuits used to detect the presence of trains on sections of rail are designed to fail in a safe manner, certain non-safe failure modes might occur, and certain conditions may exist that prevent positive detection of occupancy. Since current train control systems rely on the information provided by the track circuits to properly control the maximum speed of following trains, the integrity of this information is paramount. A wrong-side failure or a failure of track circuit resulting in an unsafe (or undetected vehicle) condition may be unlikely, but is still possible. A wrong-side failure occurs when a system, product, or component fails in an unsafe manner, or in a manner associated with a hazardous condition.
As authorities seek to maximize the safety of train passengers, it is desired to better monitor the track circuit train detection system to prevent unsafe conditions resulting from, for example, wrong-side failures of track circuits.
There is room for improvement in systems for track circuits and interlocking systems.
There is also room for improvement in methods of monitoring track circuits.